The goal of this project is to investigate two fundamental visual processing streams in the human brain, the magnocellular and parvocellular pathways. These pathways are physically segregated in subdivisions of the lateral geniculate nucleus (LGN) of the thalamus, a subcortical visual structure that connects the retina to the visual cortex. The magnocellular and parvocellular subdivisions of the LGN have different and complementary visual functions but have been difficult to study in humans due to their small size and subcortical location deep within the brain. A better understanding of the functional specialization of these pathways is important for public health, because deficits in specific subdivisions have been associated with human disorders such as dyslexia and schizophrenia. Further, it has been proposed that the magnocellular and parvocellular subdivisions preferentially provide input to large-scale visual pathways in dorsal and ventral cortex, respectively, but the precise relationships between subcortical and cortical pathway organization in the human brain remain unknown. We propose to use functional magnetic resonance imaging (fMRI) with high spatial and temporal resolution to noninvasively characterize responses to visual stimulation in human LGN. By presenting visual stimuli known to preferentially drive activity within a single subdivision, we will localize the magnocellular and parvocellular subdivisions within the LGN in each participant based on their patterns of visual responses. The localization technique we will develop may be used in the future to characterize the functional properties of the magnocellular and parvocellular subdivisions in both healthy and diseased brains. We will then investigate the functional relationships among large-scale visual cortical networks and the magnocellular and parvocellular LGN subdivisions. To do this, we will employ recent advances in imaging technology for fMRI to obtain measures of connectivity between each subdivision and each of many objectively defined cortical areas, based on correlated fMRI activity in pairs of brain regions. This investigation will help to precisely characterize relationships between subcortical and cortical visual pathways and will assess an influential but incompletely tested theory about the relative contributions of magnocellular and parvocellular pathways to visual processing in dorsal and ventral cortex. A more complete understanding of the normal functional organization of subcortical and cortical visual pathways is critical for developing treatments for multiple diseases, disorders, and injuries that affect visual areas of the brain.